1. Field of the Invention
The present invention relates to methods of manufacturing composite fiber and resin reinforcements for strength members. More particularly, the present invention relates to a process for manufacturing a fiber-reinforced composite material comprising a plurality of fibers impregnated with a polymer matrix, the composite reinforcements being relatively thin while having improved strength.
2. Description of the Prior Art
The concept of reinforcing products with fibers to strengthen the products in order to become structural members is known in the art. The advantage of doing so, the method for attachment, and conventional methods for making the structural members are also established in the art. For example, U.S. Pat. No. 5,928,735 (Padmanabhan, et al.), U.S. Pat. No. 6,179,942 (Padmanabhan, et al.), U.S. Pat. No. 5,456,781 (Tingley) and U.S. Pat. No. 6,105,321 (KarisAllen) may be considered relevant in the art. It is known that the use of composites formed by the pultrusion process is a convenient way to attain the use of fibers for reinforcing products. This is further disclosed in U.S. Pat. No. 6,037,049 (Tingley) and U.S. Pat. No. 5,362,545 (Tingley).
Pultrusion is generally defined as a continuous process of manufacturing of composite materials with constant cross-section whereby reinforced fibers are pulled through a resin, possibly followed by a separate pre-forming system, and into a heated die, where the resin undergoes polymerization and where the reinforced plastic is shaped and the resin is cured. Pultrusion is known for the ability to fabricate a continuous length of reinforced plastic and to accommodate desired placement and orientation of fibers, which allows for the mechanical properties of the pultruded part to be designed for a specific purpose or application. Pultruded parts comprise longitudinally aligned fibers for axial strength and/or obliquely aligned fibers for transverse strength. Many resin types may be used in pultrusion, including polyester, polyurethane, vinylester and epoxy.
Reinforcements for structural members have been manufactured using pultrusion processes. This process generally involves wetting fibers with resin and pulling the wet fibers through a mold where the resin is cured by heating the resin, as noted above. Exemplary pultrusion processes are disclosed, for example, in U.S. Pat. No. 2,419,328 (Watson, et al.), U.S. Pat. No. 2,684,318 (Meek), U.S. Pat. No. 3,895,896 (White, et al.), U.S. Pat. No. 5,286,320 (McGrath, et al.), U.S. Pat. No. 5,374,385 (Binse, et al.), U.S. Pat. No. 5,424,388 (Chen, et al.), U.S. Pat. No. 5,556,496 (Sumerak), U.S. Pat. No. 5,741,384 (Pfeiffer, et al.) and U.S. Pat. No. 5,783,013 (Beckman, et al.). Current known methods in the art typically do not prepare structural members having thicknesses under 0.060″, and in particular under 0.040″. In addition, the glass content of a known pultrusion process can be at about 65% by volume. Currently the fiber volume of a pultruded product based on conventional methods known in the art could range from about 10% to about 70%, depending on the particular fibers, resin, and processing technique employed.
During processing, before the resin is cured to a solid, fibers have a tendency to fracture when an uneven distribution of fibers build up in a particular area of the die. This excessive build-up of fibers results in a higher percent by volume and can result in fracture of the fiber value exceeds 70% by volume.
Another type of pultrusion process, often referred to as continuous lamination, involves spreading resin on a film, such as MYLAR®, adding fiber materials to the spread resin and then adding a top cover film to form an envelope that essentially becomes a flexible mold. This “sandwich” configuration is shaped by tension and mechanical forces, and is then pulled through an oven to cure the “sandwich” configuration into a desirable form.
A third variation of pultrusion provides placing the fibers under tension, saturating the fibers with photo-initiated resin, pulling the fibers through a series of sized dies or nip rolls to form the fibers into a bundle or web, and then exposing the fibers to high intensity ultraviolet light to initiate curing. A surface coating is often then applied and cured to provide a desired resin rich surface. This process has been used in forming artificial leather and strengthening members of fiber optic cables. Exemplary variations of this process are disclosed in U.S. Pat. No. 244,872 (Fischer), U.S. Pat. No. 4,861,621 (Kanzaki), U.S. Pat. No. 5,700,417 (Fernyhough) and U.S. Pat. No. 6,893,524 (Green).
A fourth variation of pultrusion provides placing the fibers under tension, saturating the fibers with thermo-reactive resin, and pulling the fibers through a series of sized dies to form the fibers into a round bundle while they are exposed to elevated temperatures, such as those found in an oven. This process has been used for making fishing rods, and has also been adapted for manufacturing fiberoptic cable strength members.
Thermoset polyurethane resin has shown the ability to be formulated to adjust the flexibility, elasticity and tensile properties. This elasticity has provided a more secure bonding to man-made fibers, such as aramid and nylon, as shown in U.S. Pat. No. 4,695,509 (Cordova, et al.). The adjustable formulations are highlighted in several patents that show the capabilities for making a very tough product as shown in U.S. Pat. No. 6,787,626 (Dewanjee). However, some prior art has specifically advised against using polyurethane in connection with continuously formed fiber reinforced composite strength members and the process for the manufacture thereof
Some difficulties with the current art have been identified. For example, the pultrusion process as discussed above is limited. The closed die method becomes less efficient when the thickness of the fibers is at 0.030″ and less because of the lack of space for foreign objects, crossed fibers, fiber knots and splices. This is complicated with the use of rigid resins like polyester, vinyl ester, epoxy, acrylic and others as the interlaminate shear is reduced and the product can split longitudinally (i.e., parallel to the fibers) quite easily while in process, as well as during post processing. In this instance, the processing speed will also be kept to less than 10 ft. per minute.
A method for providing a greater interlaminate shear (to reduce the splitting) in thicknesses below 0.030″ comprises adding a web of materials to the structure. The web of materials has fibers in directions other than parallel to the longitudinal fibers. In doing this, an amount of the longitudinal fibers must be replaced in a normally disproportional amount, thus reducing the product strength and requiring a thicker laminate to enable the same reinforcing function.
Many of the open (i.e., without a die or cover envelope) continuous processes have been substantially limited to the making of round products, and more specifically to the making of products having a thickness greater than 0.035″ and when substantially all of the fibers are longitudinal fibers. In addition, the more commonly used resins of polyester and vinyl ester release harmful emissions that increase when cured in an open process and are required by state and federal laws to be limited.